May 07, 2002

Packaged Treatment Plant Treats Tough Water Supplies Consistently, Economically

Since the 1960s, municipalities and industries have used
packaged water treatment plants to successfully and economically treat small
water supplies. These packaged plants have offered a smaller footprint, lower
capital cost and easy operation.

While many conventional packaged plants offer effective
treatment for raw water supplies with turbidity less than 300 NTU, raw water
supplies with higher turbidity or variable influent water quality are harder to
treat — given a low floc formation time and limited settling area in the
tubes or plates. In addition, variable quality waters require constant
monitoring of influent quality to adjust coagulant, and may require settling
aids to match the raw water conditions.

The newly developed ACTIFLOC™ packaged plant from
USFilter, Microfloc Products, addresses these problems with its unique design.
By combining enhanced microsand settling with conventional coagulation and
flocculation, the ACTIFLOC can consistently treat highly turbid, colored and/or
spiking waters before filtering with a mixed-media filter bed. The addition of
microsand allows high steady-state-solids concentration to absorb spiking
conditions, while recovery of the microsand minimizes the system’s
operating cost.

Influent water enters a coagulation chamber, where a mixer
impeller combines the influent with a coagulant for coagulation and floc
formation, similar to what occurs in other packaged treatment plants. Because
the floc that is formed will later adhere to a microsand particle, there is no
need to develop a settle-able floc which reduces coagulant requirements.

Next, flocculated water flows to the injection chamber where
another mixer impeller combines the flocculated particles with polymer and
microsand. The sand grains are effectively coated in polymer, and form a
particle many times larger than the flocculated particles, and with more
attachment sites. The polymer acts as a bonding agent between the microsand
grains and the flocculated particles. The larger sand grains covered with
polymer act as floc scavengers to collect and capture floc particles in the
flow path. Also, the larger sand grains increase the chance of floc collision,
resulting in higher clarity water.

The solids concentration of the microsand has an operating
range of 2,000 to 10,000 mg/l. The high steady-state-solids concentration
inside the unit makes it more resistant to changes in the raw water quality.

A final mixing or maturation stage ensures that the
microsand has been coated with polymer and that the flocculated particles have
been captured before the water enters the settling chamber. A mixer impeller
supplies the energy for mixing and contacting, and also keeps the particles in
suspension before the settling stage.

The final clarification stage, prior to filtration, occurs
in the settling chamber. Floc-covered sand grains rapidly settle at rates
exceeding 25 gallons per minute/square foot (61 meters per hour). Tube settlers
minimize floc carryover, and hydraulically balance the upflow of water before
collection at the top of the chamber. The floc-covered sand is collected in a
hopper bottom section of the chamber, and the settled sludge-sand mixture is
pumped out of the chamber.

As the sludge-sand mixture passes through the recirculation
pump, the polymer bond between floc particles and sand grains is broken. A
hydrocyclone at the injection chamber separates the sludge and sand. The sand
is recycled into the injection tank and the sludge is sent off for disposal.

Clarified water from the settling chamber enters a mixed
media filter section, and any floc particles that escape the clarifier are
easily captured by the filter media. The filter bed contains anthracite, filter
sand and high-density garnet to further polish the water.

During filter backwash, influent water flow is stopped,
while the clarification stage remains in operation. This prevents the microsand
from settling out, and prepares the unit for operation once the backwash is

When the unit is placed back into operation, a
filter-to-waste or rewash step is included to make sure the filter is producing
acceptable quality water before being placed back on line.

In addition to a high steady-state-solids concentration to
absorb spikes in raw water quality, the ACTIFLOC also has a coagulant
optimization control system to account for changing water conditions. A
turbidimeter continuously monitors the effluent turbidity against an
operator-selected setpoint, and adjusts the coagulant feed accordingly.

These packaged plants are pre-engineered to reduce manufacturing
lead-time, and reduce overall equipment costs. Standard sizes are available in
flow rates of 175 gpm (39.75 m3/hr), 350 gpm (79.5 m3/hr), 700 gpm (159 m3/hr)
and 1,400 gpm (318 m3/hr) per tank. Each standard plant contains two tanks, for
back-up purposes during regularly scheduled maintenance.

With the ACTIFLOC pre-engineered packaged water treatment
plant, small facilities now have a way to consistently and economically treat
their highly turbid and spiking waters, while enjoying the benefits inherent in
packaged treatment plants.

About the author

Darin St. Germain is a product manager at USFilter and is a registered professional engineer in the state of Iowa. Prior to USFilter, he was employed as a lab and operations assistant for the city of Grand Forks, N.D. water treatment plant.